This invention relates to measuring the concentration of ions implanted in semiconductor materials.
Most microelectronic devices (e.g., a microprocessors) include a series of oxide, metal, and semiconducting films grown or deposited on a semiconducting substrate. The semiconducting substrate is typically a single-crystal silicon wafer, while the semiconducting films are typically an amorphous "polysilicon" material containing small, crystalline regions of silicon. Both silicon wafers and polysilicon films are implanted with high-energy ions during fabrication so that they exhibit a specified electrical conductance. The implanted ions render silicon as being "p" or "n" type. After implantation, both silicon and polysilicon are annealed under high temperatures to heal any lattice damage resulting from the implantation process.
The electrical conductance of silicon and polysilicon is affected by three properties of the implanted ions: (1) atomic composition; (2) implantation energy; and (3) ion concentration or dose. Silicon's electrical conductivity is particularly dependent on the concentration of the implanted ions. For example, silicon wafers are typically implanted with arsenic, argon, phosphorous, oxygen, or boron ions ranging in concentration from 10.sup.10 -10.sup.16 cm.sup.-3 to improve their semiconducting properties. Polysilicon films are implanted with similar ions at higher concentrations (usually on the order of 10.sup.20 cm.sup.-3) to function as electrical conductors.
The performance of a completed microelectronic device depends critically on the electrical properties of the silicon wafer and the overlying polysilicon films, and thus the ions implanted in these materials are carefully monitored during fabrication. One property, called electrical sheet resistance, is measured by contacting a sample's surface with an electrical-testing instrument called a 4-point probe. Electrical current flowing from one probe to another depends on the resistance of the material. Resistance, in turn, varies inversely with the concentration of implanted ions.
Ion concentration is also monitored using a non-contact, optical method that excites and detects an electron-hole plasma within the sample. The response of the plasma is influenced by any damage in the semiconductor lattice resulting from the implanted ions. In this measurement, a first laser beam irradiates the semiconducting material and is partially absorbed to generate the electron-hole plasma. The plasma modifies the reflectivity of the sample's surface and can therefore be measured with a second laser beam. The reflected beam is partially reflected by the sample and then analyzed to estimate the properties of the implanted ions.
Although used throughout the microelectronics industry, both 4-point probes and instruments that measure reflectivity suffer disadvantages. 4-point probes necessarily contact the sample, and are therefore destructive. This means that these instruments can only measure "monitor" wafers or regions of "product" wafers that lack functioning devices. Instruments that monitor electron-hole plasmas by measuring reflectivity have limited use and generate signals that are difficult to interpret: they are mostly used to determine whether or not the sample has been ion implanted, rather than the actual concentration of implanted ions.